Walk into a fish shop and the tanks are labelled by a round number: 10 gallon, 20 gallon, 55 gallon. The numbers are handy names, but they are not exact volumes, and they hide the shape of the tank, which matters for the footprint on your furniture, the surface area for gas exchange and the weight on your floor. This guide gives a chart of common US tank sizes with typical outside dimensions, the volume to the brim in gallons and litres and the weight of that water. It explains how to read the chart, how the numbers are calculated and why you should always measure your own tank before you rely on any chart.
The chart
The dimensions below are typical for each name, in inches, as length × width × height. The volume is the product divided by 231 cubic inches per US gallon, and the weight uses 1 litre of water = 1 kg. These are outside-style nominal dimensions for illustration; makers vary.
| Nominal size | Typical L × W × H (in) | Cubic inches | US gallons (brim) | Litres (brim) | Water weight (kg) | Water weight (lb) |
|---|---|---|---|---|---|---|
| 10 | 20 × 10 × 12 | 2,400 | 10.4 | 39.3 | 39 | 87 |
| 20 high | 24 × 12 × 16 | 4,608 | 19.9 | 75.5 | 76 | 166 |
| 20 long | 30 × 12 × 12 | 4,320 | 18.7 | 70.8 | 71 | 156 |
| 29 | 30 × 12 × 18 | 6,480 | 28.1 | 106.2 | 106 | 234 |
| 40 breeder | 36 × 18 × 16 | 10,368 | 44.9 | 169.9 | 170 | 375 |
| 55 | 48 × 13 × 21 | 13,104 | 56.7 | 214.7 | 215 | 473 |
| 75 | 48 × 18 × 21 | 18,144 | 78.5 | 297.3 | 297 | 655 |
| 125 | 72 × 18 × 22 | 28,512 | 123.4 | 467.2 | 467 | 1,030 |
Two lessons stand out. First, the name and the arithmetic do not always match: a “20 long” works out at 18.7 gallons on those dimensions, and a “40 breeder” at 44.9. Second, the weight climbs fast, and a 125 gallon tank holds about 1,030 lb (467 kg) of water before you add glass, stand and substrate.
How the numbers are worked out
Take the 55 as an example. 48 × 13 × 21 = 13,104 cubic inches. Dividing by 231 gives 56.7 US gallons. Multiply by 3.785 for litres: 214.7. Water weighs about 1 kg per litre, so the water weighs about 215 kg, and in pounds 215 × 2.2046 = 473. Our guide to how to calculate aquarium volume walks through each step for any tank, and the Aquarium Volume Calculator does the conversion.
Why the tank is not really that full
The chart gives the volume to the brim. In use you fill to a level below the rim, and substrate, rock and equipment displace more. A realistic fill is around 90% of the brim, and after displacement, real water volume is lower again.
| Nominal size | Brim volume (US gal) | At 90% fill (US gal) | Water weight at 90% (lb) |
|---|---|---|---|
| 10 | 10.4 | 9.4 | 78 |
| 20 high | 19.9 | 18.0 | 150 |
| 20 long | 18.7 | 16.8 | 140 |
| 29 | 28.1 | 25.2 | 211 |
| 40 breeder | 44.9 | 40.4 | 337 |
| 55 | 56.7 | 51.1 | 426 |
| 75 | 78.5 | 70.7 | 590 |
| 125 | 123.4 | 111.1 | 927 |
So the “55 gallon” tank has about 51 gallons of water when filled to 90%, and less again once the substrate and decor are added. This matters for dosing treatments and for planning water changes with the Aquarium Water Change Calculator.
Long, high and breeder tanks
The same volume can come in different shapes, and shape matters:
| Shape | Footprint | Surface area | What it suits |
|---|---|---|---|
| Long | Larger | Larger | More swimming room; gas exchange across surface |
| High | Smaller | Smaller | Fits small spaces; tall plants |
| Breeder | Wide, shallow | Large | Easy aquascaping and access |
Ask a specialist what suits the fish or plants you plan to keep. The shape does not change the arithmetic, but it changes how the tank looks and how it is used.
Reading the chart in litres
Many countries use litres. For quick reference, multiply the US gallon figure by 3.785. A “10 gallon” is about 38 litres nominal, a “20 gallon” about 76 and a “55 gallon” about 208. Metric tanks are usually sold by litres, such as 60, 100, 200 and 300 litres, and dimensions in centimetres. The table below gives some common metric sizes with illustrative dimensions and the arithmetic.
| Illustrative size (cm) | Calculation | Litres | US gallons |
|---|---|---|---|
| 60 × 30 × 36 | 64,800 ÷ 1,000 | 64.8 | 17.1 |
| 80 × 35 × 40 | 112,000 ÷ 1,000 | 112 | 29.6 |
| 100 × 40 × 50 | 200,000 ÷ 1,000 | 200 | 52.8 |
| 120 × 50 × 55 | 330,000 ÷ 1,000 | 330 | 87.2 |
| 150 × 60 × 60 | 540,000 ÷ 1,000 | 540 | 142.7 |
These are examples of the arithmetic, not a list of products, since makers use many dimensions.
Using the chart to plan
- Space. Add a few centimetres around the footprint for the stand overhang, filter access and airflow.
- Floor. Check that the floor or furniture can take the weight; see how much a fish tank weighs.
- Equipment. Match filter, heater and lighting to the real volume, not the name.
- Water changes. A 25% change of a 51-gallon fill is about 12.8 gallons.
- Budget. Larger tanks cost more in water treatment, electricity and substrate.
Common mistakes
- Treating nominal gallons as exact.
- Ignoring displacement.
- Forgetting the weight.
- Mixing US and UK gallons.
- Comparing tanks only by volume, not shape.
- Skipping the measurement of your own tank.
Questions to ask in the shop
- What are the actual dimensions?
- What is the glass thickness?
- What is the real water volume?
- Is the stand rated for the weight?
- What filter is recommended for this size?
Choosing between sizes
When deciding between two sizes, think beyond price. A larger volume dilutes waste and changes in temperature more slowly, which many keepers find more forgiving, but it needs a bigger filter, more heating power, more water treatment per change and a stronger stand. A smaller tank is cheaper to run and fits more places, but changes happen faster and mistakes are less forgiving.
| Question | Smaller tank | Larger tank |
|---|---|---|
| Stability of water conditions | Changes faster | Changes more slowly |
| Cost of equipment | Lower | Higher |
| Weight | Lighter | Much heavier |
| Space needed | Less | More |
| Water per change | Less | More |
| Room for stock | Fewer, smaller fish | More choice |
A specialist shop or club can advise on the right size for the animals you want to keep.
Costs that scale with volume
Water treatments are dosed by volume, so a bigger tank costs more per change. Electricity for heating and lighting scales with size and the temperature of the room. Substrate cost scales with area. If you are budgeting, work from the real volume, and read the next guide in this series on how much a fish tank weighs to plan placement as well.
Worked check for a second-hand tank
You are offered a used tank described as “55 gallon”. Check it before you pay.
- Measure the inside length, width and height in inches. Suppose they are 47.5 × 12.5 × 20.5.
- Multiply: 47.5 × 12.5 × 20.5 = 12,172 cubic inches.
- Divide by 231: 52.7 US gallons.
- Convert to litres: 52.7 × 3.785 = 199.5.
- Water at the brim weighs about 199.5 kg (440 lb).
The tank is a 55 by name but holds 52.7 gallons to the brim, and about 47 gallons at a 90% fill. That is normal, and nothing to complain about, but it matters for treatments, filters and weight. Check the glass for scratches, silicone for damage and the base for level before buying, and ask the seller for the maker’s information.
Planning a room
Put the dimensions on paper before you buy. A 55 gallon tank at 48 × 13 × 21 inches on a stand of similar footprint needs a space of about 50 × 15 inches plus access. Leave room behind for cables and filter hoses, and above for the lid and light. The room should be away from direct sun and radiators. Check the stand’s rating, floor strength and nearby sockets, and allow room to drain and refill.
Starting small
If you are new to fish keeping, a mid-size tank is often easier to stabilise than a very small one, but a smaller tank costs less and is lighter. Take advice from a specialist shop or a local club on which fish suit which tank sizes. The tank chart gives numbers; the right fish for the tank is a separate question that depends on species, behaviour and filtration.
Converting any chart row to litres and kilograms
The chart works in gallons and pounds. To convert any row:
- Gallons to litres: multiply by 3.785.
- Litres to kilograms of water: multiply by 1 (fresh water).
- Pounds to kilograms: divide by 2.2046.
- Inches to centimetres: multiply by 2.54.
For the 55 gallon row: 56.7 × 3.785 = 214.6 litres, and 214.6 kg of water. Dimensions: 48 × 13 × 21 in is 121.9 × 33.0 × 53.3 cm. The Pet Weight Converter and Pet Measurement Converter do these conversions.
A quick comparison of footprints
Footprint matters for placement. The areas below come from the typical dimensions in the chart.
| Tank | Footprint (in) | Area (sq in) | Area (sq ft) |
|---|---|---|---|
| 10 | 20 × 10 | 200 | 1.4 |
| 20 long | 30 × 12 | 360 | 2.5 |
| 29 | 30 × 12 | 360 | 2.5 |
| 55 | 48 × 13 | 624 | 4.3 |
| 75 | 48 × 18 | 864 | 6.0 |
| 125 | 72 × 18 | 1,296 | 9.0 |
Weight per square foot of footprint rises with height: the 125 gallon tank’s water alone (about 1,030 lb) spread over 9.0 sq ft is about 114 lb per sq ft, before the stand. Use this when you compare spaces.
Quick summary
- Nominal names (10, 20, 29, 55, 75, 125) are labels, not exact volumes.
- Volume (US gal) = L × W × H in inches ÷ 231; litres = gallons × 3.785.
- A 55 on typical dimensions holds 56.7 gallons (215 litres) to the brim.
- Water weight is about 1 kg per litre, or 8.35 lb per US gallon.
- Real volume is lower after fill level and displacement.
- Use the Aquarium Volume Calculator with your own measurements.
Frequently asked questions
How many litres is a 55-gallon tank?
About 208 litres nominally (55 × 3.785), and 214.7 litres to the brim using typical 48 × 13 × 21 in dimensions.
How big is a 20-gallon tank?
Two common shapes: a 20 high at 24 × 12 × 16 in (about 19.9 gallons) and a 20 long at 30 × 12 × 12 in (about 18.7 gallons).
Why are tank dimensions different between brands?
Nominal sizes are names, not exact specifications. Makers vary dimensions a little, so always check the actual size.
How much does a 10-gallon tank of water weigh?
About 87 lb (39 kg) of water at the brim for a typical 20 × 10 × 12 in tank, before glass, stand and substrate.
What is the difference between a long and a high tank?
A long tank is wider along the front and has more surface area, a high tank is taller and shorter in length. Both can have the same nominal volume.
Do I need to subtract the substrate?
Yes, for real water volume. Substrate and decor displace water, so actual volume is lower than the table.
How do I convert gallons to litres?
Multiply US gallons by 3.785. For UK gallons, multiply by 4.546.
Sources
- NIST Guide to the SI, Appendix B.8 (exact US liquid gallon = 231 cubic inches = 3.785411784 litres)
- Typical retail tank dimensions (industry convention; makers vary slightly)
Related calculators
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Weight CalculatorWork out how much the water in a fish tank weighs so you can check your stand and floor.
- Pet Weight ConverterConvert your pet's weight between kilograms, pounds, ounces and grams.
Keep reading
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- How Much Does a Fish Tank Weigh? Full Tank WeightHow to estimate the weight of a filled fish tank: water at 1 kg per litre or 8.35 lb per US gallon, plus glass, substrate, rock and stand, with examples.